a. Field of the Invention
The present invention relates to a piston rod assembly for a drug delivery device that allows a user to select single or multiple doses of an injectable medicinal product and to dispense and deliver the set dose to a patient. In particular, the present invention relates to such drug delivery devices that are handled by the patients themselves, such as pen-type injectors.
b. Description of the Related Art
Drug delivery devices for administering single or multiple doses of an injectable medicinal product are well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Drug delivery devices of this kind have to meet a number of user specific requirements. For instance, in the case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Further, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
Typically, the medicinal product to be administered is provided in a cartridge that has a moveable piston (also commonly referred to as a “bung”, a “stopper”, or a “plunger”) mechanically interacting with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the piston in the distal direction, a certain amount of the medicinal fluid is expelled from the cartridge and may be administered to the patient by some kind of needle assembly being in fluid communication with the cartridge.
Due to inevitable manufacturing tolerances of the device and the cartridge there may for instance persist axial clearance between a cartridge's piston and the piston rod when the device is finally assembled. Typically, prior to a primary use of the device, an end-user, e.g. a patient has to conduct a so-called set-up or priming of the drive mechanism in order to ensure, that the piston of the cartridge and the piston rod are located at a pre-defined position with respect to each other, thus ensuring, that with an initial dose setting and a subsequent dose dispensing step, a predefined amount of the medicinal product can be disposed in an accurate way. By way of the set-up step, mechanical tolerances of movable components of the drug delivery device can be significantly reduced or eliminated.
Drug delivery devices are assembled in a mass production process in which for instance two housing components of the drug delivery device receive a cartridge and a drive mechanism including the piston rod, respectively. Then, in a final step of assembly, the two pre-configured housing components or respective sub-assemblies are mutually interconnected. When reaching a defined final assembly configuration, it would be beneficial for the piston rod and the piston of the cartridge to either mutually abut or to be separated by a pre-defined gap.
Exact and precise mutual positioning and alignment of the piston and the piston rod is important and crucial for accurate and reliable functionality of the drug delivery device. Moreover, the piston rod should not exert pressure to the plunger during assembly of the drug delivery device or in its final assembly configuration. Otherwise a rather uncontrolled expelling of the medicament prior to a first use of the drug delivery device may result when a needle is attached.
Since at least some or even major components of the drug delivery device are designed as plastic injection molded components, the components themselves, and their assembly are inevitably subject to certain geometric tolerances. Moreover, the cartridge itself and in particular the position of the piston within the cartridge may vary.